1. Field of the Invention
The present invention relates to an elastic wave device. More particularly, the present invention relates to an elastic wave device including a capacitive electrode that is connected in series with an IDT electrode on a piezoelectric substrate and that includes a comb-shaped electrode pair.
2. Description of the Related Art
Various elastic wave devices that use elastic waves, such as surface acoustic waves or boundary acoustic waves, have been used. An elastic wave device includes a piezoelectric substrate and an IDT electrode formed on the piezoelectric substrate. The elastic wave device provides, for example, a resonant characteristic and a filter characteristic by using elastic waves excited at the IDT electrode.
Meanwhile, to improve the characteristics of the elastic wave device, an additional capacitor may be provided. More specifically, for example, as described in Japanese Unexamined Patent Application Publication No. 8-65089, a capacitor may be connected in parallel with an IDT electrode that forms a ladder surface acoustic wave filter device, or a capacitor may be connected in series with the IDT electrode. If the capacitor is provided in the elastic wave device, as described in Japanese Unexamined Patent Application Publication No. 8-65089, it is known that the capacitor is formed on a piezoelectric substrate and a comb-shaped electrode pair, in which electrodes are interdigitated with each other, forms a capacitor. Since the capacitor is formed by the comb-shaped electrode pair, a facing area of the electrodes can be large, and hence a large electrostatic capacity can be obtained.
As described in Japanese Unexamined Patent Application Publication No. 8-65089, if the comb-shaped electrode pair (hereinafter, referred to as “capacitor-forming comb-shaped electrode pair”) formed on the piezoelectric substrate forms the capacitor, elastic waves may be unintentionally excited at the capacitor-forming comb-shaped electrode pair. Hence, to prevent characteristics of the elastic wave device from being degraded as a result of the unintentionally generated elastic waves, the capacitor-forming comb-shaped electrode pair is preferably provided such that an intersecting width direction of the capacitor-forming comb-shaped electrode pair is perpendicular to an intersecting width direction of the IDT electrode.
However, referring to FIG. 12, if the number of pairs of electrode fingers of a capacitor-forming comb-shaped electrode pair 101 is increased to obtain a large electrostatic capacity, the capacitor-forming comb-shaped electrode pair 101 becomes long in an intersecting width direction of the IDT electrode, and hence a large unused space 103 may be formed on a piezoelectric substrate 102.
In contrast, referring to FIG. 13, if an intersecting width of a capacitor-forming comb-shaped electrode pair 101 is increased to obtain a large electrostatic capacity, unlike the state shown in FIG. 12, the capacitor-forming comb-shaped electrode pair 101 becomes long in the intersecting width direction, and hence a large unused space is substantially eliminated. Accordingly, the elastic wave device can be decreased in size. However, if the intersecting width of the capacitor-forming comb-shaped electrode pair 101 is increased, a resistance loss of the capacitor-forming comb-shaped electrode pair 101 is increased, and hence the characteristics of the elastic wave device may be degraded. More specifically, if the elastic wave device is, for example, an elastic wave filter device, an insertion loss may be increased.